On change mtu the driver compares between hardware queried mtu and
software requested mtu. We need to compare between software
representation of the queried mtu and the requested mtu.

Fixes: facc9699 ('net/mlx5e: Fix HW MTU settings')
Signed-off-by: Doron Tsur <doront@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>